This application is a national phase of PCT/FR99/02804 which was filed on Nov. 16, 1999, and was not published in English.
The present invention relates to a method for writing and reading on an information medium comprising a recording layer with a succession of zones of material with at least a first and second physical state respectively.
xe2x80x9cPhysical Statexe2x80x9d implies a particular structure or composition of the material linked to a physical property that is characteristic of the state.
As an example, a physical state may imply particular doping of the material, a given ferroelectric structure or a crystalline structure of the material. The state of the material results in an electrical or optical property, such as a characteristic resistivity.
In particular, the recording layer may comprise a succession of regions in a crystalline and amorphous state respectively.
In general the invention has applications in recording information. For example, the information may be in the form of digital data, images or sounds.
In particular, the invention may be used in the fields of television and creating computer memories.
FIG. 1 is a schematic drawing of a system for writing and reading on an information medium according to a known technique based on the distinction between two physical states of a material.
The information medium bears the general reference number 1.
It comprises a substrate medium 10 covered with a layer of an electrical conductor material that constitutes an electrode 12.
A recording layer 14 covers electrode 12. For example, this layer is of a material such as Ge2Sb2Te5 that is in either a crystalline or an amorphous state.
Information is recorded in the recording layer using coding in a succession of zones in a first physical state (crystalline) and zones in a second physical state (amorphous).
In the figure the amorphous zones bear the reference 14a and the crystalline zones reference 14c. 
The information is recorded by the zones becoming more or less heated such that they change from an amorphous to a crystalline state or vice versa.
The heating is achieved, by means of an electric current that is more or less intense in the various zones of recording layer 14. The current flows between a conductor point 20 applied against a write/read surface of the recording layer and electrode 12. The current is supplied by a generator (not shown in the figure). Point 20 and information medium 1 are displaced relative to one another so as to scan recording layer 14.
This recording technique is called xe2x80x9cphase change recordingxe2x80x9d.
The recorded information on medium 1 is also read using conductor point 20. It uses the electrical properties of the recording layer material that has a different resistivity in the amorphous and the crystalline states.
A source of voltage 22 is connected between conductor point 20 and electrode 12 such that a measuring circuit is created with a zone of the recording layer with which point 20 is in contact.
A current carried by the measuring circuit is detected by ammeter measuring means 24 and conductor point 20 is displaced on the surface of the recording layer in order to scan it.
Measuring current I may have two values depending on whether the zone of recording layer 14 in contact with point 20 is amorphous 14a or crystalline 14c. The resistance of recording layer 14 has two values respectively Ra and Rc depending on the amorphous or crystalline state of the material.
In practice, resistance Ra is stronger than resistance Rc, resulting in the following:
Ia=V/Ra if the recording layer is amorphous,
Ic=V/Rc if the recording layer is crystalline.
In these expressions V is the value of the voltage supplied by source of voltage 22.
The higher the Ra/Rc relation, the easier it is to distinguish between these two states.
The above measuring method poses problems linked to the mechanical and electrical contact between the conductor point and the write/read surface of recording layer 14. After a certain number of reading operations the displacement of point 20 against recording layer 14 causes mechanical wear of the point and the recording layer. The wear results in reading errors or inaccuracy.
FIG. 2 shows a second known technique for reading an information medium 1, such as that described, that prevents the problems of mechanical wear mentioned above.
The reading technique uses the tunnel effect.
The equipment used to implement the second reading technique is more or less the same as that described with reference to FIG. 1.
It comprises a data medium 1 consisting of a substrate 10, an embedded electrode 12 and a recording layer 14. The reading apparatus comprises a conductor point 20, a source of voltage 22 and measuring means 24. However, unlike the apparatus in FIG. 1 conductor point 20 is held away from the surface of recording layer 14 by a distance d. Despite this distance a current is still capable of penetrating the recording layer by means of the tunnel effect.
A current-voltage diagram is used to determine the value of the intensity of the current. This type of diagram is shown in FIG. 3. In this diagram voltage V is shown on the abscissas and current I on the ordinates. The current-voltage characteristic is a curve with an elbow, as in tunnel diodes. Three characteristics C1, C2, C3 are plotted in FIG. 3. They show three different distances, d1, d2, d3 respectively, between point 20 and the write/read surface of the recording layer. For the same voltage applied between conductor point 20 and electrode 12, the smaller the distance between the point and the read surface the greater the current carried by the measuring circuit.
In the example shown, distances d1, d2, d3 with characteristics C1, C2, C3 are such that d1 less than d2 less than d3.
In the diagram load lines are defined that relate current I to voltage V and that slope depending on the more or less resitive character of the circuit. When the tested zone is amorphous its resistance Ra is great and the drop in voltage is significant. In a crystalline zone resistance Rc is lower and the drop in voltage smaller. The load lines are therefore as shown in FIG. 3 with references DCa and DCc for an amorphous zone 14a and a crystalline zone 14c respectively.
If characteristic C2 is considered relative to a distance d2 the point of operation of the measuring circuit is located at either Ma (if the zone is amorphous) or Mc (if the zone is crystalline). The current-voltage couple is then either Ia-Va (amorphous zone) or Ic-Vc (crystalline zone).
The variables Va, Vc, Ia and Ic are the voltages and currents respectively in amorphous zones 14a and crystalline zones 14c. 
Measuring means 24 are used to measure currents Ia or Ic in this second read mode to distinguish the physical state (amorphous or crystalline) of the scanned zones in order to read the information encoded on the recording medium.
The second read mode does not cause wear on the write/read surface of the recording layer or on the micro-point. It does, however, have the drawback of being extremely sensitive to the measurements of distance d separating the micro-point from the recording layer.
As shown in FIG. 3, for a given voltage the current carried in a given zone of the recording medium (amorphous or crystalline) is greatly dependent on 10 distance d (characteristics C1, C2, C3).
Therefore, a crystalline zone could very easily be mistaken for an amorphous zone if distance d between the point and the write/read surface were to be increased.
An error of this kind is likely to occur frequently if distance d is small and the recording medium is not perfectly flat.
The second read mode is also therefore subject to measuring errors.
The aim of the present invention is to provide a recording medium and a method for reading the recording medium that does not have the difficulties and limits of the read techniques described above.
One particular aim is to provide a reading method that does not cause any wear of either the read equipment or recording medium.
Another aim is to provide a reliable reading method that excludes reading errors linked to variations in the distance between the read equipment and the recording medium.
Another aim is to provide an improved information medium suited to the reading method.
Yet another aim is to provide a method for recording on the information medium.
In order to achieve these aims, the invention relates more precisely to a method for reading an information medium comprising a recording layer with a succession of zones of material with distinct first and second physical states of the material respectively. According to the invention the recording layer is scanned with means for detecting electrical fields.
The method of the invention is based on detecting the existence of a measurable electrical field correlated to the succession of zones of a solid material in its different physical states. xe2x80x9cPhysical statexe2x80x9d of the material implies a state linked to the structure or composition of the material as compared to a state linked to an electrical charge, i.e. independent of the structure or composition of the material.
The difference of physical state may be intrinsic, such as a difference of the crystalline or ferroelectric state of the material. The recording layer may therefore comprise a succession of zones having respective crystalline and amorphous states of said material. It may also comprise a succession of zones with a first and second ferroelectric state respectively.
The difference of physical state may also be extrinsic, i.e. obtained by a modification of the composition of the material. The recording layer may, for example, be a layer of semi-conductor material with a succession of zones with a first and second state of distinct doping respectively.
The recording layer may be made of a silicon material or II-IV or III-V compounds. It may also be an alloy with a base of tellurium (Te), and/or germanium (Ge), and/or antimony (Sb), and/or silver (Ag), and/or indium (In), and/or copper (Cu), and/or chrome (Cr), and/or vanadium (V) and/or selenium (Se).
Finally, the material of the recording layer may be a PZT-type ferroelectric material, i.e. Pb, Zr, Ti oxide alloy.
The reading method may use an apparatus for detecting electrical fields that comprises a read head and means for imposing a periodic oscillatory movement on the point. With an apparatus of this kind the reading results from the detection of modifications in the oscillatory mode.
In one version the apparatus may also be equipped with means for applying electrostatic force to the point. In this event the reading results mainly from the detection of variations in the force applied to the point.
The invention also relates to an information medium comprising a recording layer with a succession of zones of material in a first amorphous state and a second crystalline state respectively. According to the invention the information medium may also comprise a resistive protective layer covering one write/read surface of the recording layer. The resistivity of this layer is lower than the resistivity of the material of the recording layer in each of the physical states.
Due to its low resistivity the protective layer does not prevent information from being written in the various zones by modifying its crystalline or (amorphous) state by means of an electric current.
Furthermore, according to the method of the invention the protective layer does not constitute an obstacle for reading whereas said layer would render the reading methods of the prior art inoperative.
The information medium may also comprise an electrical conductor layer that constitutes an electrode and that covers a surface of the recording layer facing the write/read surface.
Finally, the invention also relates to a method for writing encoded information on an information medium, as described above.
The method comprises writing phases during which a conductor writing point is applied to a writing face of the information medium in order to create localised heating using electricity that is sufficient to cause a phase change of the material in a zone of the recording layer. The method also comprises displacement phases during which the writing point is remote from the writing surface of the information medium and during which a relative displacement of the point and the information medium is effected in order to position the point opposite a new zone of the recording layer.
According to one particular implementation of the method, when the point is applied to the reading surface a current is sent through the recording layer, the current being selected (depending on the encoded information) between a first value that establishes a zone of the recording layer in an amorphous state and a second value that establishes a zone of the recording layer in a crystalline state.
The writing current sent through the recording layer is caused by applying a voltage between the conductor point and the conductor electrode on the surface facing the reading surface. The writing current may have a low initial value that slightly heats the material of the recording layer and renders it locally crystalline. On the other hand, the second, higher value heats the material to a higher degree and renders it amorphous.
In order to write encoded information on a recording medium that is initially amorphous the second value of the current may be chosen to be zero (except to erase).
The point is considered to be applied to the write/read surface when the point is in direct contact with the recording layer (when said layer is not provided with a protective layer) or when the point is in contact with said protective layer (when the recording layer is provided with such a layer).
The writing method in which the point is remote during the relative displacement of the support and point minimises the wear on these components.
According to one version of the writing method the writing point and an electrode in contact with a surface opposite the writing surface may also be used as capacitor plates to cause capacitive heating of a zone of the recording layer.